tty: n_gsm: initialize more members at gsm_alloc_mux()

syzbot is reporting use of uninitialized spinlock at gsmld_write() [1], for
commit 32dd59f96924f45e ("tty: n_gsm: fix race condition in gsmld_write()")
allows accessing gsm->tx_lock before gsm_activate_mux() initializes it.

Since object initialization should be done right after allocation in order
to avoid accessing uninitialized memory, move initialization of
timer/work/waitqueue/spinlock from gsmld_open()/gsm_activate_mux() to
gsm_alloc_mux().

Link: https://syzkaller.appspot.com/bug?extid=cf155def4e717db68a12 [1]
Fixes: 32dd59f96924f45e ("tty: n_gsm: fix race condition in gsmld_write()")
Reported-by: syzbot <[email protected]>
Tested-by: syzbot <[email protected]>
Cc: stable <[email protected]>
Acked-by: Jiri Slaby <[email protected]>
Signed-off-by: Tetsuo Handa <[email protected]>
Link: https://lore.kernel.org/r/[email protected]
Signed-off-by: Greg Kroah-Hartman <[email protected]>

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